*{
    margin: 0;
    padding: 0;
}
html,body{
    height: 100%;
    background-image: linear-gradient(
        to top ,skyblue,pink
    );
    display: flex;
    justify-content: center;
    align-items: center;
}
.box{
    display: flex;
    justify-content: center;
    align-items: center;
    width: 500px;
    height: 300px;
    border: 1px dashed #000;
}